1. Field of the Invention
The present invention relates to a voltage-current converter for providing a constant current from the voltage generated in a constant voltage source.
2. Description of the Prior Art
To improve mobility and allow battery drive, many types of equipment use semiconductor circuits driven by low voltage, especially, integrated circuits which contain analog parts and digital parts mixed on the same substrate.
These semiconductor circuits often require a voltage-current converter to generate constant current, which is used for bias circuits such as an op-amp and a comparator, from voltage generated by the constant voltage source, which uses, for example, a band gap reference circuit.
In the below description, a conventional voltage-current converter is explained with reference to FIG. 11. One example of the conventional voltage-current converter is shown by reference numeral 100 in FIG. 11. Constant voltage Vr based on VSS generated in a constant voltage source 90 is supplied to the reversing input of op-amp 103. Output Vg of op-amp 103 is supplied to the gate of P-channel of two MOS (metal-oxide semiconductor) transistors MP101 and MP102 which have the same channel-width and the same channel-length (hereafter, referred to as "same W/L").
Both sources of these MOS transistors MP101 and MP102 are connected with a high-level power-supply voltage VDD. The drain Vp of MP101 is connected with the one side of resistance R and capacitor C10 connected in parallel, and connected with non-reversing input of op-amp 103. The other side of resistance R and capacitor C10 connected in parallel is connected with VSS which is the low-level power-supply voltage. The drain of MP102 is connected with constant current output terminal Io.
In FIG. 11, 110 shows a basic composition of an op-amp (operation amplifier) comprising CMOS (complementary MOS) formed on the NMOS substrate. This op-amp 110 is supplied with the bias current Ib from the constant current output Io of the above-mentioned voltage-current converter 100.
Next, the operation of the conventional voltage-current converter 100 is explained. The output of op-amp 103 which comprises the voltage-current converter 100 is positively fed back through MP101, and operates as shown by the following equation 1. EQU Vr=Vp (equation 1)
The current I1 which flows through the resistance R is shown by equation 2. EQU I1=Vp/R (equation 2)
Because the same gate voltage Vg is supplied to MP101 and MP102, which are of same W/L, drain currents I2 and I1 of MP101 and MP102 are equal to each other. Therefore, equation 3 results. EQU I2=I1 (equation 3)
The (equation 4) is obtained by (equation 1)-(equation 3). EQU I2=Vr/R (equation 4)
Thus, conventional voltage-current converter 100 generates constant current I2 from constant voltage Vr.
The above-mentioned voltage-current converter had the following faults. Voltage-current converter 100 in FIG. 11 oscillates easily because it uses the positive feedback operation of op-amp 103. The purpose of connecting capacitor C10 with resistance R connected with drain of MP101 in parallel is to maintain the stability operation of op-amp 103 by suppressing this oscillation. Moreover, because the resistance of resistance R and W/L of MP101 and MP102 are fixed when resistance R is built into LSI, the value of the constant current is fixed. In sum, a variable current was not able to be obtained.